In addition to having a fuel cell, conventional fuel cell systems have one or more power sources such as a battery pack, an ultracapacitor, etc. Each one of these power sources can be used to carry out a number of operations within the fuel cell system.
Generally, the battery pack is connected to the system at the output of the fuel cell stack. It is used to both start the operation of the fuel system, as well as to share the load of the associated application (e.g., vehicle electrical system). The ultracapacitor is typically connected across the battery pack to provide peak power in the form of burst power pulses to the application powered by the fuel cell system.
In addition to powering the application to which it is connected, the fuel cell stack is also utilized to charge the battery pack and/or the ultracapacitor, when their respective states of charge drop below a predetermined level.
Generally, these power sources have differing rated voltage outputs. The voltage outputs of the battery pack and ultracapacitor are typically below the system voltage. Accordingly, for the battery pack and ultracapacitor to provide power to the auxiliary systems of the fuel cell system, and to provide the necessary burst power pulses to the load of the system, the voltage of these power sources needs to be stepped up, or boosted. Alternatively, when the voltage level of the battery pack and/or ultracapacitor are low, therefore necessitating a “re-charge” by the fuel cell stack, the voltage level of the fuel cell must be stepped down, or “bucked.”
To carry out these boost and buck functions, it is known to provide separate bi-directional converters to connect the battery pack and ultracapacitor, respectively, to the system bus. The use of separate converters for each power source results in added components and hardware to a system, thereby increasing size. This is a problem for applications that are constrained in size. Under this conventional approach, each time an additional power source is added to the system, at least one additional converter is required, thereby further adding additional components and circuitry to the system, increasing size as well as cost.
It is the objective of the present invention to provide a device that minimizes or eliminates one or more of the foregoing problems.